This invention relates to a method of suspension polymerization of vinyl chloride, and more particularly, to an improved method of substantially eliminating or reducing polymer scale adhesion on the inner surfaces of the polymerization vessel and auxiliary equipment used in combination therewith.
As used herein, the term "vinyl chloride" shall include vinyl chloride itself and also a monomeric mixture in which vinyl chloride forms a major component and other monomers copolymerizable therewith being a minor component.
Vinyl chloride is suspension polymerized on an industrial scale in an aqueous medium containing a suspension agent and an oil-soluble catalyst. The suspension polymerization of vinyl chloride is accompanied by deposition of polymer scales on the inner surfaces of the polymerization vessel and auxiliary equipment, such as agitation blades, baffle plates, and supply inlet, used in connection with the polymerization.
Polymer scales deposited on the inner surfaces of the polymerization vessel and auxiliary equipment decrease the efficiency of heat conduction of the vessel and equipment, decrease the polymer yield, and interfere with the properties of the polymer product since the scales peeled off from the surfaces of the vessel and equipment may become mixed with the polymer product. Moreover the scales which accumulate on the surfaces must be periodically removed; this incurs expenditure of time and labor and shutdown of the equipment for such scale removal. Accordingly, the plant cannot be efficiently utilized.
In an attempt to eliminate these deficiencies, different methods have been proposed for preventing scales from forming on the polymerization vessel wallsand auxiliary equipment surfaces, such as, for example, the walls and surfaces have been coated with a variety of different compounds. In U.S. Pat. No. 3,669,946 there is disclosed a coating compound selected from the group consisting of nitrogen atom containing organic compounds, comprising azo radical containing compounds, nitro radical containing compounds, nitroso radical containing compounds, azomethine radical containing compounds, azine cyclic containing compounds and amines; sulfur atom containing organic compounds comprising thiocarbonyl radical containing compounds, thioether radical containing compounds and thioalcohol radical containing compounds; oxygen atom containing compounds comprising quinones, ketones, aldehydes, alcohols having at least six carbon atoms and carboxylic acids having at least six carbon atoms. U.S. Pat. No. 3,778,423 suggests use of a free radical inhibitor.
However, although the method of U.S. Pat. No. 3,669,946 can reduce the amount of scales adhering to the vessel surfaces, the reduction or prevention of scale deposition is not substantial and much more is necessary for the method to be industrially feasible. The method of U.S. Pat. No. 3,778,423 is deficient in that the polymerization velocity is decreased due to a large amount of free radical inhibitors being used; thus it is not always satisfactory. As is apparent from the foregoing discussion, a coating compound has not yet been discovered which substantially, economically and commercially and satisfactorily prevents deposition of polymer scales on the surfaces of the polymerization vessel and auxiliary equipment used in connection therewith.